Who is the Arsonist?

Assuming Yu Fusheng was indeed the mystery person, nearly everything could be explained other than two of the incidents. The first was Wu Fangfang and Jiang Ke's scandalous photos. Those photos were an obvious set up used by the mystery person to bait Zhao Yu into visiting Jiang Ke. But where did Yu Fusheng get those photos? 

Just to be sure, Gao Facai from Forensics had reexamined the photographs and determined their authenticity. With current technology, if the photographs were fake, it may not have been easy to tell. But, the photo of Wu Fangfang was ancient and printed on old paper, so it was definitely verifiable!

At the seafood market, Jiang Ke had mentioned the photographs were used as extortion and he hadn't known that Wu Fangfang was a police officer, so he never followed up. Even though he had been vague about the whereabouts of the photographs, there was no doubt they were with Wu Fangfang.